1. Field of the Invention
This invention relates to apparatuses and methods for improvement of radio transmitter and receiver frequencies of a local radio communication unit that is communicating digital data with a remote communication unit.
2. Description of Related Art
Communication systems often comprise a plurality of local units such as radiotelephone handsets that communicate digital data by radio transmissions with a remote unit such as a cellular phone base station. The radio frequencies of the communication channels and frequency error tolerances for transmissions on the channels are typically specified by regulatory rules. For example, for the GSM mobile telephony standard [Ref. 1], the frequency tolerance is specified to be 0.05 ppm for the base station and 0.1 ppm for the handset. The frequency tolerances ensure that the level of radio interference between channels is tolerable and that accurate data demodulation is possible at the local unit and the remote unit. In the base station the transmitter and receiver radio frequencies are typically phase locked to very stable reference oscillator signal available in the base stations in order to meet the radio frequency tolerances specified by regulations. However, the cost of such stable reference oscillators are typically prohibitive for the handsets, so that provision for accurate transmitter and receiver frequencies in the local unit at lowest possible cost is important.
In a local communication unit the conventional solution for accurate radio frequencies is the use of a relatively low cost voltage controlled crystal oscillator (VCXO) to serve as a reference oscillator wherein the oscillator frequency is approximately linearly related to the magnitude of a VCXO control voltage. The transmitter and receiver radio frequencies are phase locked to the VCXO oscillator frequency, but the frequency error in parts per million (ppm) of the transmitter and receiver, or equivalently the frequency error of the VCXO, is not sufficiently accurate with a free running VCXO to meet frequency tolerance specifications. Therefore, the VCXO control voltage is adjusted based on estimated radio frequency error of the receiver in accordance with well known feedback control principals such that the radio frequency errors of the receiver and transmitter are sufficiently reduced by feedback control principals. This methodology for radio frequency control in the local unit is a conventional automatic frequency control (AFC) loop. With modern electronics the analog VCXO control voltage is generated by a digital-to-analog converter (DAC), and the digital feedback control signal applied to the DAC is produced by AFC digital control logic.
A drawback associated with the conventional AFC loop is that the achievable accuracy of the normalized VCXO frequency in ppm, or equivalently the normalized transmitter and receiver RF frequencies in ppm derived therefrom is limited by the digital-to-analog converter (DAC) quantization error. The DAC step size or quantization error associated with a least significant bit change in the DAC control signal represents the frequency control resolution of the VCXO. The associated VCXO frequency quantization error is an irreducible systematic bias error and not a random error reducible by averaging or filtering in the AFC tracking loop.
The magnitude of frequency quantization error is dependent primarily on two parameters, the number of bits N represented in the DAC and the characteristic slope of the VCXO. The characteristic slope S of the VCXO is defined as the ratio of the normalized frequency change in units of parts per million (ppm) to change in the control voltage. Let B equal the voltage control span of an N-bit DAC. Then the VCXO frequency quantization error q in Hz/LSB is given byq=S*B/2NGenerally, a lower cost VCXO requires a larger frequency swing per volt to correct the larger frequency errors; i.e., a lower cost VCXO tends to have a larger value for the characteristic slope S and hence larger VCXO quantization error. Therefore, with the conventional AFC loop, usage of a lower cost VCXO is constrained by the maximum tolerable level of DAC quantization error which is related to radio frequency tolerance specifications. This is an important constraint because the VCXO typically represents a major cost component in a local unit such as a radio telephone.
As a numerical example of VCXO frequency quantization error induced by the VCXO DAC, the NDK5411B [Ref. 2], which is a high quality VCXO suitable for radio telephone usage, has a characteristic slope of S=15 ppm per volt. A typical DAC implemented in modem integrated circuit technology has a voltage span of B=2 volts with N=10 bits of resolution, and the resultant VCXO frequency quantization error is q=0.03 ppm/LSB. The VCXO frequency quantization error represents a substantial fraction of the frequency tolerance of 0.1 ppm allowed for local units under GSM specifications so that there is very little margin for usage of a lower cost VCXO having a larger quantization error. This example teaches how VCXO quantization error is an important factor in the cost of the local unit.
Another drawback of the conventional AFC is susceptibility of the VCXO frequency to DAC switching noise in addition to the DAC quantization step when the ADC digital control word changes. A common method for mitigation of DAC switching noise is the introduction of a filter with a long time constant, for example one millisecond, between the VCXO and its controlling DAC. This has the drawback that such filtering in the VCXO controller DAC response can significantly extend the settling time of the VCXO in recovery to its original reference frequency immediately after termination of sleep mode. Sleep mode is an interval of inactivity between scheduled communication transactions when all inactive components of the local unit, including VCXO, are powered off to conserve battery life for portable local units. Near termination of sleep mode, the VCXO is powered back on with scheduled allowance for oscillator settling time. Thus, a longer settling time implies a reduction in the power down interval with a consequent reduction in the battery savings. Sleep mode is commonly used in portable communication units such as pagers and cellular phones during idle intervals in standby mode.
Another deficiency of the conventional AFC loop is that the local unit processing of data bursts exhibit large timing errors and VCXO frequency errors when the local unit is powered on. AFC tracking loops are typically inefficient in reducing such large initial errors and for this reason, before the AFC tracking loop is closed, a prior acquisition phase is typically employed to reduce the large initial time and frequency errors to sufficiently small values that efficient closed loop AFC tracking can be switched in and accurate data demodulation performed. The reduction of timing error is a synchronization function which is performed by conventional means and not described in this disclosure. The acquisition time, or duration of the acquisition phase, represents a wasted overhead interval before track mode when accurate data communications is possible so that acquisition time must be reduced to the smallest possible value.
In the local unit local oscillator (LO) signals are generated to serve as mixer signals for frequency upconversion and downconversion. Typically, a subsystem of phase locked loop (PLL) frequency synthesizers and voltage control oscillators (VCO) generate the LO signals which are phase locked to the VCXO signal [Ref. 3]. The PLL/VCXO subsystem and the frequency downconversion and upconversion operations are configured so that the local unit transmitter and receiver radio frequencies have zero frequency error on the assigned channels when the VCXO is operating at its specified nominal frequency. At initiation of acquisition mode, the normalized VCXO frequency error in units of ppm, or equivalently the normalized RF frequency errors of the transmitter and receiver in ppm, have many error contributors, but the two most dominant sources are the VCXO temperature characteristic and aging. The temperature characteristic is a reflection of temperature sensitivity of the crystal and components of the oscillator circuit. Aging is VCXO frequency error caused by aging of the VCXO crystal.
Factory calibration determines the nominal control voltage for the VCXO at which the specified nominal frequency of the VCXO is achieved at a specified nominal temperature T0, where typically T0=25 degrees C. After calibration, the VCXO frequency will deviate from the specified frequency over a specified operating temperature range and the temperature sensitivity is the specified tolerance on VCXO frequency over the specified temperature range. The drift rate of VCXO frequency due to aging of the VCXO crystal is typically given in units of ppm per year. As an example a high quality temperature compensated VCXO suitable for a radio telephone handset, the NDK5411B [Ref. 2], has a specified frequency of 13.000 MHz with a temperature sensitivity specification of +/−2.5 ppm over a temperature range of −20 degress C. to +75 degress C., with an aging specification of +/−1.0 ppm per year.
The cost of the VCXO increases as specifications on temperature sensitivity and aging are tightened. A commonly used technique for reduction of VCXO temperature sensitivity is the integration of a temperature compensation circuit with the VCXO oscillator circuit. The compensation circuit corrects or compensates for the VCXO temperature characteristic by integration of a compensating circuit with the oscillator circuit. The compensating circuit generates an incremental adjustment to the VCXO analog control voltage such that variation of VCXO frequency with temperature is reduced over the specified temperature range. The additional cost of the compensation circuit increases with compensation accuracy. For this reason, an acquisition algorithm with a greater tolerance for VCXO temperature sensitivity and aging facilitates the use a lower cost VCXO or reduction of the acquisition time.
A low cost VCXO with little or no temperature compensation typically has a large temperature sensitivity specification in which the VCXO temperature characteristic, the VCXO frequency error as a function of temperature, exhibits large frequency error swings that can be approximated by a third order polynomial. That is, for temperature T, and nominal temperature T0, The frequency error f can be approximated as                                  f          =                      p            ⁡                          (                              T                -                T0                            )                                                                    =                      a0            +                          a1              ⁡                              (                                  T                  -                  T0                                )                                      +                                          a2                ⁡                                  (                                      T                    -                    T0                                    )                                            2                        +                                          a3                ⁡                                  (                                      T                    -                    T0                                    )                                            3                                          where typically the first two terms of the polynomial p dominate except for large deviations of temperature from nominal temperature T0 by greater than +/−25 deg. C. For small temperature changes, the change in VCXO frequency can be approximated by the slope of the polynomial temperature characteristic p. Under dynamic temperature conditions the VCXO temperature varies slowly and approximately linearly so that the VCXO frequency change in dynamic temperature conditions can be approximated by a ramp in frequency over any time interval comparable to the AFC loop time constant. Therefore, as is known to those of ordinary skill in tracking loop design a Type 2 or higher order AFC loop is very effective in accurately tracking out ramp frequency perturbations caused by dynamic VCXO temperature conditions.
Another method for VCXO temperature compensation is software temperature compensation wherein the compensation algorithm runs in a general purpose processor such as a DSP or microcontroller. A circuit with a temperature sensor such as a diode or thermistor generates a voltage that is a known function of temperature. An analog-to-digital converter (ADC) converts the sensor voltage to a digital value held in the ADC register according to a known ADC conversion rule. The processor then reads the ADC register and translates the digital ADC reading to temperature by the known temperature sensor characteristic and ADC conversion characteristic for example by means of a lookup table in processor memory. The calculated temperature is then converted to a compensating VCXO DAC digital increment by means of a lookup table or polynomial that represents the inverse of the VCXO temperature characteristic. The compensating digital increment is then added to the nominal VCXO control signal. Software temperature compensation is often attractive in systems where the processor is available at no additional cost for temperature compensation because the processor is already present to perform other functions for the local unit. In track mode a drawback to software temperature compensation is that a sensor having sufficient accuracy to provide the fine frequency quantization error needed in tracking mode is a major cost component and a temperature sensor ADC with a very high resolution of approximately 16 bits is needed. However, coarse software temperature compensation in acquisition mode is a useful and cost effective method of reducing VCXO frequency error in acquisition mode.
A Doppler shift due to changing range between local unit and remote unit may also contribute to radio frequency errors. With the conventional AFC loop where the frequency error estimates are based on the received signal the AFC loop tracks the received signal so that the receiver frequency error is driven to zero but the resultant transmitter frequency error will be twice the Doppler shift. For systems where the Doppler shift is significant provisions are typically provided for measurement of the frequency error of the received signal at the remote unit and for transmission of a correction command to the local unit. The local unit then responds by adjustment of the local unit transmission frequency to null the measured Doppler error.
In U.S. Pat. No. 5,542,095 by Petranovich, a system for frequency tracking is described that utilizes the conventional methodology in which a nonlinear AFC algorithm obtains estimated receiver frequency error and controls the VCXO control voltage by means of a nonlinear feedback control signal. Petranovich does not mention ramifications of VCXO frequency quantization errors. However, Petranovich's disclosure does acknowledge the possible occurrence of signal distortions or switching noise caused when the VCXO ADC command changes value. One object of Petranovich's invention is restriction of the updating of the VCXO frequency only to intervals between telephone calls in order to avoid such signal distortions. A disadvantage of this strategy of fixing the VCXO ADC level during a call is that any VCXO frequency drift due to temperature change would not be tracked out by the AFC loop and could result in accrual of a very large frequency error that exceeds radio frequency error tolerances specified by regulatory rules. Temperature changes during a call may, for example, be caused by thermal flow on the printed circuit board from the power amplifier to the VCXO. During a call, say of duration 10 minutes, the frequency change of a VCXO in a radiotelephone can easily exceed the 0.1 ppm frequency tolerance allowed for example by the GSM specifications if the VCXO DAC is fixed during the call interval.
U.S. Pat. No. 5,740,525, Apr. 14, 1998, by J. H. Spears, utilizes a conventional AFC loop wherein the VCXO is controlled by a nonlinear AFC feedback algorithm to minimize receiver error during tracking mode. The Spears invention has no mention of the ramifications of VCXO DAC quantization error. The Spears invention provides for software temperature compensation in the initial acquisition phase to reduce large initial VCXO frequency error. In the tracking phase software compensation is not used, and the AFC algorithm steps the VCO in successive steps to reduce the frequency error to a predetermined magnitude.